1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, and more specifically to a semiconductor device manufacturing method for making it possible to easily and quickly detect a filling-defective of an aluminum-based alloy filled in a hole immediately after the aluminum based alloy has been filled into the hole under a high temperature process.
2. Description of Related Art
With an advanced microfabrication of semiconductor devices, the size of a through hole connecting between an upper level interconnection and a lower level interconnection is becoming increasingly smaller. The upper level interconnection and the lower level interconnection are ordinarily formed of an aluminum-based alloy (Al-based alloy) by a sputtering process. If the aspect ratio (hole depth/hole diameter) of the hole size exceeds about 0.8 as the result of the advanced fine patterning (down-sizing of the hole diameter), the step coverage of the Al-based alloy in the hole lowers, so that a connection defect often occurs. As a result, reliability of the interconnection deteriorates, or conduction defects occur, with the result that reliability of the semiconductor device becomes a problem. In order to overcome this problem, it has been proposed to deposit Ti (titanium) or TiN (titanium nitride) having a step coverage better than that of the Al-based alloy, before the Al-based alloy is filled into the hole, for the purpose of ensuring an electrical connection. However, this method results in an increased number of process steps.
Now, some filling-up processes for improving the step coverage of the Al-based alloy filled into a fine hole in order to overcome the above mentioned problem, will be described with reference to the drawings. First, a high temperature Al-sputtering, which is one of these processes, will be described with reference to FIGS. 1A to 1C.
As shown in FIG. 1A, a first interconnection layer 3 is formed and patterned on an insulating film 2 formed on a silicon substrate 1, and an interlayer insulating film 4 is formed on the first interconnection layer 3 and the insulating film 2 by means of a CVD (chemical vapor deposition) process. Then, through holes 5 are formed in the interlayer insulating film 4 to reach the first interconnection layer 3 by a lithography and a dry etching. After a Ti (titanium) film 7 having a good reactivity to an Al-based alloy is uniformly formed on the interlayer insulating film 4 and in the through holes 5 as shown in FIG. 1B, an Al-based alloy layer 14 is deposited in the same vacuum atmosphere by sputtering the Al-based alloy at a low temperature of not greater than 150.degree. C. by a thickness corresponding to about one third to a half of a desired thickness, and then by sputtering the Al-based alloy at a high temperature of 400.degree. C. to 500.degree. C. by the remaining thickness of the desired thickness, thereby to completely fill the through holes 5 with an Al-based alloy layer 14 as shown in FIG. 1C. In this process, Ti and the Al-based alloy react with each other to form a reaction layer 91 on an inner wall surface of the through hole.
However, in the case of filling the through hole 5 with the Al-based alloy 14 by the high temperature sputtering, a filling-defective hole 15 having a void 12 within the through hole as shown in FIG. 2 often occurs. This filling-defective is generated if the flowability of the Al-based alloy 14 lowers because of deterioration of the degree of vacuum and a drop of a heater temperature in a deposition machine, the Al-based alloy grows to have an overhang in the contact hole, so that a mouth or an upper port of the hole is closed by the overhang, with the result that an upper portion of the hole is closed at an initial stage of the filling-up process.
If the filling-defective hole 15 occurs, the reliability of the interconnection drops, similar to the drop of the step coverage of the Al-based alloy in the hole. Therefore, generation of the void in the hole must be avoided. In addition, if the filling-defective occurred, it is important to detect the existence of the filling-defective.
However, the existence of the filling-defective cannot be found out even if the upper portion of the hole is observed, because the upper portion of the hole is plugged or covered with the Al-based alloy 14 as shown in FIG. 2. In the prior art, therefore, it was necessary to cleave a reference wafer (which is different from a wafer on which products are actually produced) immediately after the filling-up process is completed in the reference wafer, and to observe a plane of cleavage by a SEM (scanning electron microscope) in order to find out the filling-defective. But, various problems were encountered in this method. For example, it is difficult to observe many holes, and it is actually impossible to cleave a product chip after the filling-up process is completed. In addition, it is difficult to accurately compare the reference wafer to the actual product wafer, and the cost increases.
In order to avoid the above mentioned problems, Japanese Patent Application Pre-examination Publication No. JP-A-06-069307 has proposed a non-destructive method for detecting the filling-defective of the Al-based alloy in actual products. Now, this method will be described with reference to FIGS. 3A and 3B.
FIGS. 3A and 3B illustrate a defective detecting and observing pattern which is formed together with the patterns of products on a wafer and which is used for detecting a defect of the Al-based alloy filled into the contact hole. An underlying metal layer 7 is formed to cover a surface of an interlayer insulating film 4 formed on a silicon substrate 1 and bottom and side surfaces of through holes formed to penetrate through the interlayer insulating film 4. This underlying metal layer 7 is formed to have a triple layer structure of Ti/TiON/Ti, in order to prevent aluminum from spearing into the substrate.
Specifically, FIG. 3A illustrates the defective detecting and observing pattern after the Al-based alloy (AlSi: aluminum silicide) is filled into the hole. In a good filled hole 17, an AlSi film 18 and an Al--Si--Ti film 19 (which is a reaction product of AlSi and Ti of the uppermost layer of the underlying metal layer 7) is formed in the hole and as a lowermost layer of an interconnection layer. On the other hand, in a filling-defective hole 20, no reaction product 19 is generated, and therefore, a non-reacted Ti remains in the uppermost layer of the underlying metal layer 7.
On the other hand, after the product is completed, the defective detecting and observing pattern is exposed by a lithography, and only the Al-based alloy 18 is removed by a wet etching, as shown in FIG. 3B. After the Al-based alloy 18 is removed, the defective detecting and observing pattern is observed by the SEM or other means. The Al--Si--Ti remaining in the good filled hole 17 has a large concavo-convex surface, and on the other hand, the non-reacted Ti remaining in the filling-defective hole 20 has a smooth surface. This difference in surface condition can be discriminated from a surface morphology.
According to the above mentioned method, it is possible to non-destructively detect whether or not the filling-defective exists in the product.
However, the above mentioned method requires the steps of exposing only the defective detecting and observing pattern by the lithography and of etching the Al-based alloy, in order to detect the filling-defective. As a result, the number of process steps is disadvantageously increased, and therefore, the manufacturing cost is correspondingly increased.
In addition, since a considerable time elapses until the detection of the filling-defective is performed, even if the filling-defective is found out, there is possibility that the filling-defective has occurred in products for which the Al filling has been completed until the detection of the filling-defective is performed. Namely, a damage to the products is magnified. This is because it is not possible to detect the filling-defective immediately after the Al filling is completed.